Somedays, I still think Charlotte should have refused to part with the locket (with hindsight of S3). Other days, I think neither deserved it because Gracie wouldn't have wanted it to belong to anyone, especially with either girl with the way they behaved.
The lockets are hugely symbolic in the context of the show, and it really is a fascinating topic.
The lockets were made when Gracie, Julia, and Louise became mermaids, a symbol of their friendship that is tied to their secret. When Gracie discards her locket, it’s because she doesn’t want to be a mermaid anymore and does give it all up later.
The show is so focused on the locket and who deserves it that it forgets one thing: Gracie, who grew up. She had a life, raised a family. She had other aspects of her life that didn’t involve being a mermaid, something that she gave up so that she could have that life. She treasured that period of her life, telling stories about mermaids and passing on her case of drawings, but it didn’t define her.
So, on the one hand, I think Charlotte should’ve been allowed to keep something that belonged to her grandmother, because she loved her so much and clearly still misses her. But on the other hand, Gracie didn’t want it. She didn’t want her locket, she didn’t want her tail, she didn’t seem to regret giving it all up. So, what did that locket mean to her, really, in the long run? Nothing. Sure, Gracie only liked one-of-a-kind pieces, but I think that’s more about Max, and her own independent nature. Honestly, I don’t think she would’ve cared about Charlotte getting the locket.
Symbolically, that locket is tied to being a mermaid, but the three lockets were also made as a set, for three friends. Which… fits Cleo more than Charlotte. Season three notwithstanding, those lockets are meant to symbolize their friendship, and that’s more about Cleo.
What I do think is that Charlotte should’ve come to this conclusion herself instead of being told by Lewis she has to give it back, and then actually gives it back herself. This could’ve lent to a conversation between Cleo and Charlotte that was a long time coming, and could’ve given a lot of character development.